1. Field of the Invention
The present invention pertains to a system and method for monitoring a metabolic parameter of a user.
2. Description of the Related Art
It is well known to monitor the oxygen consumption or oxygen uptake of an individual for purposes of monitoring the physiologic condition of that person. The phrases “oxygen uptake” and “oxygen consumption” are used synonymously, and are both represented by the expression “VO2” or, for simplicity “VO2”. Oxygen consumption is a measure of the amount of oxygen that the body uses in a given period of time, such as one minute. It is typically expressed as milliliters of oxygen used per kilogram of body weight per minute (ml/kg/min). Measuring the rate of oxygen consumption is valuable, for example, in anesthesia and intensive care situations because it provides an indication of the sufficiency of a patient's cardiac and pulmonary function. VO2 can also be used to monitor the fitness of an individual or athlete.
VO2 is conventionally calculated as the difference between the volume of oxygen inspired and the volume of oxygen expired. The standard or direct calculation of VO2 is given by the following equation:VO2=Vi*FiO2−Ve*FēO2,  (1)where: “VO2” is oxygen consumption, “Vi” is inspired volume, “FiO2” is the inspired oxygen concentration, “Ve” is the expired volume, and “FēO2” is the mixed expired oxygen concentration.
An alternative method of calculating VO2 uses only the expired breath volume, Ve. In this scenario, the inspired breath volume Vi is calculated (rather than measured) based on the assumption that the nitrogen volume is the same for both inspired and expired gas, which is usually true because nitrogen is not consumed or produced by the body. This is referred to as the nitrogen balance. The calculation of Vi, rather than measuring it, also assumes that the effect of temperature and humidity are the same for both inspired and expired gas volumes.
This modification of equation (1), which uses a calculation of Vi based on the nitrogen balance noted above, is known as the Haldane transform. According to this technique, Vi is calculated as follows:Vi=Ve*FēN2/FiN2,  (2)where “FēN2” is the concentration of expired nitrogen, and “FiN2” is the concentration of inspired nitrogen. Based on this, the Haldane transform becomes:Vi=Ve*(1−FēCO2−FēO2)/(1−FiCO2−FiO2),  (3)and the oxygen consumption calculation becomes:VO2=Ve*[FiO2*((1−FēCO2−FēO2)/(1−FiCO2−FiO2))−FēO2],  (4)where “FēCO2” is the expired carbon dioxide concentration, and FiCO2 is the inspired carbon dioxide concentration.
Calculating VO2 using the Haldane transform has the advantage that the effects of errors in volume measurements that are not “common mode” are eliminated, because only the expired volume measurement is used. Common mode errors are errors that effect both the Vi and Ve measurements, such as a calibration error in a flow sensor. Assuming, of course, the same sensor is used to measure Ve and Vi.
Conventional CO2 sensor technology is generally capable of very fast and accurate measurement of airway CO2 and can be sufficiently robust to track changes in respiratory CO2 over long periods of time. It is desirable to measure respiratory oxygen with similar speed and accuracy, so that one or more parameters such as oxygen consumption, energy expenditure, respiratory quotient, or related metabolic measurements can be accurately assessed. This can be used for various applications such as respiratory spirometry, for example.
Although mainstream oxygen sensing technology that is potentially fast and accurate enough to use for such applications is becoming available, speed and robustness may be inferior to available CO2 sensing technology. Estimations of oxygen consumption or metabolic parameters derived from measurements of respiratory gasses can only be accurate if all the gas measurements are sufficiently accurate. A method of determining if airway conditions interfere with the measurement of oxygen is also needed to ensure that a corrupted waveform is not used to calculate oxygen consumption or other metabolic parameters. Given that fast, accurate, and robust oxygen measurement may be the limiting factor in attaining respiratory measurements for oxygen consumption or metabolic assessment, methods to help identify corrupted oxygen waveforms and to correct such waveforms are desired.
It should also be appreciated, however, that the present invention can be applied to sensing of other gasses, and is not limited to measurements of oxygen or carbon dioxide.